This application is a national stage application of International Application No. PCT/EP2012/057453 (WO 2012/150149 A3), filed Apr. 24, 2012 which is herein incorporated by reference in its entirety.
1. Field of the Invention
The invention relates to a laser machining system in which the brilliance of a machining laser beam can be adjusted.
2. Background
Hitherto, different machining processes have frequently been covered by separate laser machining systems, whereby for applications such as, for example, welding, radiation of medium brilliance (typical beam parameter product (BPP)≧8 mm•mrad) is preferably used, and for applications such as, for example, cutting, radiation of high brilliance (typical BPP≦4 mm•mrad) or, for example in remote cutting, even highest brilliance (BPP≈0.4 mm•mrad) is preferably used. High-power laser systems which require only one laser and permit rapid (<100 ms) switching between two or even three different radiation types have hitherto not been possible because of a lack of optical components in the high-power range (>2 kW), such as, for example, beam switches. In addition to the radiation types for the machining of materials, the beam qualities of the laser sources can also be divided into three ranges, which are generally subject to different physical limits:                single-mode range (beam quality factor M≦1.5 or BPP<2 about 0.4 mm•mrad)        few-mode range (1.5<M2<6 or 0.4<BPP<2 mm•mrad)        multimode range (M2≧6 or BPP≧2 mm•mrad).        
The closest dual-brilliance laser machining system 10 shown in FIG. 1 has two different laser sources, namely a first laser source 11 with high-power multimode radiation (HP-MM) and a second laser source 12 with high-power single-mode radiation (HP-SM). These two radiations with their different brilliancies are fed to a laser machining head 13 via transport fibres 14 and can be used electively as a machining laser beam 15. In addition to the disadvantage of high operating and installation costs, the laser machining system 10 also has the disadvantage of the limited lengths of transport fibres in the high-power range for single-mode radiation (<10 m) and also for few-mode radiation (<100 m), These arise owing, to non-linear effects in the transport fibre, which occur to an increased extent at high powers. Multimode radiation in the high-power range, on the other hand, can be transported almost without loss over the transport distances of 100 m required in industry.
FIG. 2 shows an alternative dual-brilliance laser machining system 20 which has only one laser source 21 with high-power multimode radiation (HP-MM). Two separate conventional transport fibres 22 are connected to two laser outputs of the laser source 21, so that the power is fed via one of the two transport fibres 22 to the laser machining head 23. In the laser machining head 23 itself, one of the two transport fibres 22 is simply guided through and thus constitutes a fibre output with the brilliance of the laser source 21. The other transport fibre 22, on the other hand, is attached to an ytterbium fibre oscillator 24, which improves the beam quality of the laser source significantly and thus provides a fibre output with radiation of high or highest brilliance. However, stepless mechanical switching between the two transport fibres 22 is not possible, and it is also only relatively slow (typically 100 ms). The machining laser beam leaving the laser machining head 23 is denoted 25.
The dual-brilliance laser machining system 30 shown in FIG. 3, which has a laser source 31 with high-power multimode radiation (HP-MM) and a fibre-integrated optical beam switch 32, functions in a similar manner. The high-power multimode radiation (HP-MM) of the laser source 31 is fed to the beam switch 32, which is located upstream of the laser machining head 33, by a conventional transport fibre 34. The beam switch 32 is able to switch (<10 ms) the radiation between two fibres 35 of identical construction without changing the quality of the beam. One fibre 35 is simply guided through the laser machining head 33 and thus constitutes a fibre output with the brilliance of the laser source 31. The other fibre 35 is attached to an ytterbium fibre oscillator 36, which improves the beam quality of the laser source significantly and thus provides a fibre output with radiation of high or highest brilliance. The machining laser beam leaving the laser machining head 33 is denoted 37.
Optical beam switches for multimode radiation are available commercially. These beam switches are free-beam solutions which cannot, however, be used for single-mode radiation. The switching times in these beam switches are between 50-100 ms, and the power capability is greatly dependent on the quality of the brilliance preservation between the input and the output fibre. Starting from radiation of high brilliance it is possible to couple by fibre-to-fibre coupling to a fibre with a greater beam parameter acceptance, so that radiation of medium brilliance is generated by mode excitation.
FIG. 4 shows a dual-brilliance laser machining system 40 which has a laser source 41 with high-power single-mode radiation (HP-SM) and a fibre-integrated optical beam switch 42. The single-mode radiation is implemented from the single-mode transport fibre 43 in the laser machining head 44 on fibres 45 with different beam parameter acceptance. Using this principle, each of the three radiation types mentioned above can be generated from the laser source 41. Disadvantages are, however, both the very short single-mode transport fibre (<10 m) and the extremely high power densities in the beam switch 42. The machining laser beam leaving the laser machining head 44 is denoted 46.
FIG. 5 shows a further commercial dual-brilliance laser machining system 50, which has a laser source 51 with high-power multimode radiation (HP-MM) and a multi-clad transport fibre 52. The radiation of the laser source 51 is coupled into either the inner fibre core 52a or an outer fibre core 52b which surrounds the inner fibre core 52a annularly, in order to obtain radiation with different brilliancies in the laser machining head 53. The machining laser beam leaving the laser machining head 53 is denoted 54. For cutting applications, the laser radiation is coupled into the inner fibre core 52a, which is typically 100 μm in size, of the multi-clad transport fibre 52. For welding applications, a suitable wedge is additionally introduced into the free propagation beam of the laser beam. The resulting beam shift upstream of a focusing lens effects a displacement of the focal point from the inner fibre core 52a of the multi-clad transport fibre 52 into the outer fibre core 52b which, with an outside diameter of 400 μm to 600 μm, is far larger. Two different brilliances can thus be selected from only one transport fibre by simple and also rapid switching. However, owing to the free propagation beam coupling, this, principle can be used in the high-power field only in the multimode range.